Allow jumbo frames on node experiment links

It's complicated.

We cannot just allow this on all node types because not all of our switches are configured to pass jumbo frames (by which we mean 9k-ish bytes) and changing them to support that now could have fidelity effects for existing experiments. So we would only do this (on the mothership) for the z9500 switch (d430s) where, after consultation with Dell engineers, we are reasonably confident that there will be no bad side effects. We already have the MTU cranked up on the Arista as well, so the d820s will be able to do it.

We will have to evaluate what we do on all the CloudLab clusters, but Emulab is the immediate concern so that we can get jumbos from Powder radios to Emulab backend nodes.